1. Field of the Invention
The invention relates generally to industrial earth-boring cutters and, more particularly, to the bearing system and attachments therefor for earth-boring cutters.
2. Background Art
Industrial earth-boring cutters, such as the type used in raise bore and shaft-drilling assemblies are well known in the art. An industrial earth-boring cutter 1, as shown in FIG. 1, typically comprises a central journal assembly 2 on which a cutter body 3 is rotatably mounted. The cutter body 3 typically includes ribs, protuberances, or hard inserts 4 to break up and crush a formation 5 when the cutter body 3 is pressed against and rolled over the formation 5.
The cutter 1 shown in FIG. 1 is a raised bore cutter. A ball bearing 10 and roller bearings 11 are disposed between the journal assembly 2 and the cutter body 3 to allow the cutter body 3 to rotate freely with respect to the journal assembly 2. The ball bearing 10 is usually provided to carry axial load, and the one or more roller bearings 11 are typically provided to carry radial loads. In this configuration the roller bearings 11 are placed around the journal assembly 2 prior to sliding the journal assembly 2 into the cutter body 3. Then the ball bearing 10 is put into place by inserting bearing balls through the ball hole 13 in the journal 2. Once the bearing balls are in place, a ball plug 12 is inserted into the ball loading hole 13 and then a ball plug retainer 14 is inserted into the journal 2 to retain the ball plug 12 in place.
To prevent damage to the bearing balls of the ball bearing 10 and edges of the ball loading hole 13, cutter designs known in the art have the ball hole 13 placed at 180 degrees from the load bearing zone of the journal assembly 2. This placement is selected to prevent forcing the bearing balls against the rough edges of the ball loading hole 13 as they pass over the hole 13. If the ball loading hole 13 were positioned in the load bearing zone, the bearing balls would forcibly impact the edges of the ball loading hole 13, probably resulting in metal chips and debris being removed from the journal 2 so as to contaminate the lubricant and eventually destroy the bearings and seals.
Once assembled, the cutter 1 is typically attached to a rotatable headplate (not shown) by a support bracket 6 or similar structure. Typically the support bracket 6 includes a base attachable to the rotatable headplate (not shown) and legs 7 on each side of the base extending away from the base. Each leg 7 includes a yoke 8 at its distal end which is configured to receive and fixably couple to a support shaft 9 of the journal assembly 2 which extends axially outward at each end of the cutter 1.
For many applications, industrial cutters are limited by the bearing capacity or bearing life. A major cause of bearing failure in industrial cutter systems is spalling of the non-rotating journal bearing surface. Spalling is the flaking off of material from a surface. Spalling of the non-rotating journal bearing surface is the result of a fatigue process caused by the rolling elements as they passed across the position the journal surface that carries the load. For example, as the rolling elements roll across the journal surface, the surface is repeatedly loaded and unloaded, which initiates subsurface cracks that ultimately cause spalling. When the journal surface spalls, hard steel debris contaminates the lubricant which causes rapid wear and damage to the rest of the operable bearing and seal components which eventually results in bearing failure.
Ideally, the load-bearing journal surface should be replaced with a new surface before it spalls so that the life of the bearing can be increased. This may be accomplished by rotating the journal during servicing of the cutter to place the previously unloaded journal surface in the load bearing position. One cutter design which allows for rotation of the journal by 180 degrees is shown in FIG. 2. However, this design uses cylindrical roller thrust bearings instead of ball bearings. In this design, the ball bearing (shown at 10 in FIG. 1) is substituted by a plurality of small roller bearings 20 transversely disposed between the journal assembly 2 and the cutter body 3 along opposed upper and lower paths defined between a projection 21 extending from the journal surface and an internal recess 22 formed in the cutter body 3. Because this design has no ball bearing, concerns regarding the placement of the ball loading hole (13 in FIG. 1) are eliminated. Therefore, it is possible to reverse the journal to expose a previously substantially unloaded surface as a replacement surface before significant spalling of the first load-bearing surface takes place. However, this cutter configuration requires very tight tolerances on four different axial bearing surfaces to maintain good control of axial loading and deflection. A closely toleranced cone bearing sleeve 23 is also necessary to assemble the thrust elements of the bearing. This sleeve 23 greatly restricts the outer bearing diameter, however, which limits radial roller bearing capacity.
In prior art cutter designs which use ball bearing retention, as previously explained, the ball loading hole is placed 180 degrees from the load zone. While this configuration ensures little or no load on the ball loading hole, this design does not allow for rotation of the journal. Therefore, the substantially unloaded surface of the journal bearing in these designs can not be later used during the cutter life. Further, if the journal were rotated, it would put the rough opening of the ball loading hole into a position of maximum radial loading, which would lead to premature bearing failure as described above.
It is desirable to have a simplified cutter which uses ball bearing retention and permits rotation of the journal so that a previously substantially unloaded surface may be subsequently used to carry load while maintaining the ball loading hole in a position outside of the load bearing zone so that the life of the bearing may be increased.
The invention is a rotary cutter mount for an earth-boring cutter. The mount includes a bearing journal adapted to be coupled to a cutter body. The bearing journal has a rotary cutter body rotationally coupled to an exterior bearing surface of the journal. A first mounting end of the bearing journal is shaped to enable rotationally fixed positioning in a corresponding yoke. The yoke is operatively coupled to the body of the earth-boring cutter. A ball race is formed in an exterior surface of the bearing journal, and a ball loading passage is formed in the bearing journal. The ball loading passage has an exit hole on the ball race. The exit hole is positioned so that a rotary orientation of the exit hole is disposed in a rotary orientation which is at a selected angular displacement from a direction of maximum radial loading on the bearing journal. A shape of the first mounting end of the journal and a shape of the corresponding yoke are adapted to enable mounting in a plurality of rotary orientations. Each of the selected rotary orientations is such that the exit hole is oriented other than in the direction of maximum radial loading.